This invention relates to a flat liquid crystal display device, and more particularly to a flat display device having a plurality of display elements which are defined by liquid crystal cell portions formed between the scanning and the signal electrodes arranged in the form of a matrix and the method of driving the flat display device. In particular, the present invention relates to a driving method which is effective for the improvement of the display quality in the flat display device.
One method for driving a flat display panel is shown in Japanese Patent Laid-Open No. 38935/78 and Japanese Patent Laid-Open No. 52686/83. This method discloses a driving circuit connected to one end of transparent electrodes in order to drive a display panel.
However, in the above-mentioned prior art, since large capacity dot matrix type liquid crystal panels are required in many applications, the transparent electrodes increase in length and width. The electrode resistance, R, and capacitance, C, across the terminal of the driving circuit to the end of the electrode increases. This increase results in degradation of the display. For example, when the liquid crystal panel has 640.times.400 dots and is driven at a duty ratio of 1/200, R=10.about.60 K.OMEGA., and C=800.about.2000 pF.
When the driving waveform changes, there is a delay period of up to several tens of .mu.sec. in which the display elements stabilize. This delay time is significant relative to one scanning period (60.about.80 .mu.s). Due to the delay time, the effective driving voltage applied to the respective display elements varies during stabilization from the predetermined value obtained by the voltage standard method. As a result, unevenness of color contrast occur, as shown in FIG. 2. The quality of the display is reduced so much that it may be difficult to distinguish the non-selected and selected regions.
FIG. 2 shows one embodiment of the prior art wherein unevenness of color contrast is generated between the non-selected regions 11 and 12 when every other horizontal lines is ON. As shown in the upper side of the portion 11, when there are a large number of signal electrodes 14 and every other horizontal line is in the ON state, the display is too light. In the upper side of the portion 12, when there are fewer signal electrodes 15 and every other horizontal line is in the ON state, the display is too dark. The color contrast is likely to be more even when the resistance of the scanning electrodes is high.
Increasing the thickness of the electrodes in order to reduce the resistance is ineffectual because of the inferior alignment and increased cost of the panel due to the reduction of the throughput in the manufacturing. Therefore, there is a limit to the thickness of the transparent electrodes that can be achieved.
In order to eliminate the above problems, the object of the present invention is to drive the electrodes of the liquid crystal panel from both terminals thereof, thereby providing a flat display device possessing a high quality display and little unevenness of contrast.